Capillary electrophoresis has been applied widely as an analytical technique because of several technical advantages: (i) capillaries have high surface-to-volume ratios which permit more efficient heat dissipation which, in turn, permit high electric fields to be used for more rapid separations; (ii) the technique requires minimal sample volumes; (iii) superior resolution of most analytes is attainable; and (iv) the technique is amenable to automation, e.g. Camilleri, editor, Capillary Electrophoresis: Theory and Practice (CRC Press, Boca Raton, 1993); and Grossman et al, editors, Capillary Electrophoresis (Academic Press, San Diego, 1992). Because of these advantages, there has been great interest in applying capillary electrophoresis to the separation of biomolecules, particularly in nucleic acid analysis. The need for rapid and accurate separation of nucleic acids, particularly deoxyribonucleic acid (DNA) arises in the analysis of polymerase chain reaction (PCR) products and DNA sequencing fragment analysis, e.g. Williams, Methods 4:227-232 (19920; Drossman et at, Anal. Chem., 62: 900-903 (1990); Huang et at, Anal. Chem., 64:2149-2154 (1992); and Swerdlow et at, Nucleic Acids Research, 18:1415-1419 (1990).
Since the charge-to-frictional drag ratio is the same for different sized polynucleotides in free solution, electrophoretic separation requires the presence of a sieving medium. The initial sieving media of choice were gels, but problems of stability and manufacturability have led to the examination of non-gel liquid polymeric sieving media, such as linear polyacrylamide, hydroxyalkylcellulose, agarose, and cellulose acetate, and the like, e.g. Bode, Anal. Biochem., 83:204-210 (1977); Bode, Anal. Biochem., 83:364-371 (1977); Bode, Anal. Biochem., 92:99-110 (1979); Hjerten et at, J. Liquid Chromatography, 12:2471-2477 (1989); Grossman, U.S. Pat. No. 5,126,021; Zhu et al, U.S. Pat. No. 5,089111; Tietz et al, Electrophoresis, 13:614-616 (1992).
Another factor that complicates separations by capillary electrophoresis is the phenomena of electroendoosmosis. This phenomena, sometimes referred to as electroosmosis, is fluid flow in a capillary induced by an electrical field. It has impeded the application of capillary electrophoresis to situations where high resolution separations are required, such as in the analysis of DNA sequencing fragments. The phenomena arises in capillary electrophoresis when the inner wall of the capillary contains immobilized charges which cause the formation of a mobile layer of counter ions which, in turn, moves in the presence of an electrical field to create a bulk flow of liquid. Unfortunately, the magnitude of the electroendoosmotic flow can vary depending on a host of factors, including variation in the distribution of charges, selective adsorption of components of the analyte and/or separation medium, pH of the separation medium, and the like. Because this variability tends to reduce ones ability to resolve closely spaced bands analyte, many attempts have been made to directly or indirectly control such flow. The attempts have included covalent modification of the inner wall of the capillary to suppress charged groups, use of high viscosity polymers, adjustment of buffer pH and/or concentration, use of a gel separation medium covalently attached to the capillary wall, and the application of an electric field radial to the axis of the capillary, e.g. Hayes et at, Anal. Chem., 65:2010-2013 (1993); Drossman et al (cited above); Hjerten, U.S. Pat. No. 4,680,201; Van Alstine et al, U.S. Pat. No. 4,690,749; Wiktorowicz et al, Electrophoresis, 11:769-773 (1990); Belder et at, J. High Resolution Chromatography, 15:686-693 (1992).
Most of these approaches have met with mixed success or have only been used in the separation of analytes quite different chemically from nucleic acids. In particular, the use of capillary gels for DNA separations have been hampered by manufacturing problems and problems of stability and reliability during use, e.g. Swerdlow et al, Electrophoresis, 13:475-483 (1992).
In view of the strong scientific and industrial interest in being able to conveniently and accurately separate a variety of biomolecules, particularly polynucleotides, it would be desirable to have available a low viscosity electrophoretic separation medium capable of suppressing electroendoosmotic flow and of reducing analyte-wall interactions.